An irreversible cycle model of a Diesel heat engine is established and used to investigate the influence of multi-irreversibilities, which mainly result from the non-isentropic compression and expansion processes, finite rate heat transfer processes and heat leak loss through the cylinder wall, on the performance of the cycle. Expressions for the power output and efficiency of the cycle are derived by introducing the pressure ratio and the compression and expansion efficiencies. The characteristic curves of the power output and efficiency, varying with the pressure ratio, and the power output versus efficiency are obtained. It is found from the curves that there are optimal values of the pressure ratio at which the power output and efficiency attain, respectively, their maxima. Moreover, the effects of the various irreversibilities and the highest temperature of the working substance on the performance of the cycle are discussed in detail. The optimum criteria of some important parameters, such as the power output, efficiency and pressure ratio, are obtained, and consequently, the optimal operating region of an irreversible Diesel heat engine is determined. (c) 2006 Elsevier Ltd. All rights reserved.